If there is an endless debate in SF, it’s about the capability of humans to imagine an alien world, with alien species. At least at a philosophical level, it seems an impossible task – how to imagine something we have not the mental structures to conceive, let alone clues about the way it can present itself? There are some astrobiologists thinking, with some reasons, that we may even not recognise an alien form of life should we be stumbling upon it – simply because it has a shape or a functioning we don’t normally associate with life, or we are not able to detect. Plants, for example, might not be green at all (See: http://www.scientificamerican.com/article/the-color-of-plants-on-other-worlds), and there is no way to know how they would look like on another planet: provided they do exist, of course.
So it is quite interesting that, when we witness in nature something – it happens relatively often – defying any logic, our normal reaction is to define it “alien”. A misnomer, certainly, but that speaks volumes about what we are expecting from aliens. A recent example is, again, given by plants – and when I say “plants” I refer to real ones and not, or not only, to the so-called Cryptids, exotic organisms only existing in myth or literature and whose existence has not been proved (so far).
Keeping to the realm of the proven, there are some frightening examples nonetheless. The Giant Hogweed, commonly found in the UK, that burns you raw at the point you can’t expose your flesh to the sunlight for years (http://www.dailymail.co.uk/news/article-2408578/Walker-Keith-Copper-stepped-poisonous-giant-hogweed-Whitley-Bay.html); the Devil’s Claw, innocuous but looking like an alien, nasty bugs you want to keep away from your feet (http://www.cracked.com/article_18979_10-creepy-plants-that-shouldnt-exist.html); and the New Zealand Nettle Tree, one of the most poisonous organisms in the world, that has already killed a human just by contact (http://www.nzpcn.org.nz/flora_details.aspx?ID=1354). Not bad, indeed.
Shifting now to the previously mentioned cryptids – well, horror is the limit. You have all you can fear, from man-eating tree (http://www.museumofhoaxes.com/hoax/archive/permalink/man_eating_tree_of_madagascar) to vampire vines (http://www.futilitycloset.com/2011/03/06/the-devils-snare). This recent article, http://mysteriousuniverse.org/2014/05/carnivorous-cryptid-plants-of-the-world, is a good starting point, full of inspiring illustrations.
Thus it’s not surprising SF authors have made good use of this tropos, and one of the classics here is certainly John Wyndham’s The Day Of TheTriffids, where the Earth scourged by a mysterious plague that make people blind is also invaded by a frightening species of plant.
Japanese SF, in its peculiar form of anime and manga, has been receptive to the concept too. Many of us Westeners still remember Mazone, an alien species of plant-shaped women created by Leiji Matsumoto in Captain Harlock fictional universe (http://www.cornponeflicks.org/harlock/harlockmain.html). Nice looking, for sure, but not less deadly than the nettle tree. Actually more.
New (real) species are still being discovered, and it seems there is no end to natural wonders, no matter how alien they could look to us. (What about a metal-eating plant? Help yourself: http://www.utahpeoplespost.com/2014/05/rare-metal-eating-plant-species-rinorea-niccolifera-discovered-in-philippines).
That’s good news for everybody, including movie producers, always preoccupied with finding suitable locations for their exoplanet scenarios (http://clarkesworldmagazine.com/blevins_12_11): with such a variety of creepy organisms, they are truly spoilt for choice!